1. Field of the Invention
The present invention relates to a method for efficiently providing a pilot signal in environments in which antennas are added to an existing system in a Multiple-Input Multiple-Output (MIMO) communication system.
2. Discussion of the Related Art
LTE Physical Structure
3rd Generation Project Partnership (3GPP) Long Term Evolution (LTE) supports radio frame structure type 1 applicable to Frequency Division Duplex (FDD) and radio frame structure type 2 applicable to Time Division Duplex (TDD).
FIG. 1 illustrates radio frame structure type 1. Radio frame structure type 1 is comprised of 10 subframes each consisting of two slots.
FIG. 2 illustrates radio frame structure type 2. Radio frame structure type 2 is comprised of two half frames, each of which consists of five subframes, a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS). Each of these subframes consists of two slots. The DwPTS is used for initial cell search, synchronization, and channel estimation at a User Equipment (UE). The UpPTS is used for channel estimation and uplink transmission synchronization of the UE at a Base Station (BS). The GP is used to remove interference occurring in uplink due to a multipath delay of a downlink signal between the uplink and a downlink. Meanwhile, one subframe consists of two slots regardless of a type of a radio frame.
FIG. 3 illustrates a slot structure of an LTE downlink. As shown in FIG. 3, a signal transmitted in each slot may be described by a resource grid including NRBDLNSCRB subcarriers and NsymbDL Orthogonal Frequency Division Multiplexing (OFDM) symbols. NRBDL represents the number of Resource Blocks (RBs) in a downlink slot, NSCRB represents the number of subcarriers in one RB, and NsymbDL represents the number of OFDM symbols in the downlink slot.
FIG. 4 illustrates a slot structure of an LTE uplink. As shown in FIG. 4, a signal transmitted in each slot may be described by a resource grid including NRBULNSCRB subcarriers and NsymbUL OFDM symbols. NRBUL represents the number of RBs in an uplink slot, NSCRB represents the number of subcarriers in one RB, and NsymbUL represents the number of OFDM symbols in the uplink slot.
A Resource Element (RE) is a resource unit defined as an index (a, b) in the uplink slot and the downlink slot and represents one subcarrier and one OFDM symbol. Here, ‘a’ is an index on a frequency domain and ‘b’ is an index on a time domain.
FIG. 5 illustrates a downlink subframe structure. A maximum of three OFDM symbols of a front portion of a first slot within one subframe corresponds to a control region to which a control channel is allocated. The remaining OFDM symbols correspond to a data region to which a Physical Downlink Shared Channel (PDSCH) is allocated. Examples of downlink control channels used in a 3GPP LTE system include a Physical Control Format Indicator Channel (PCFICH), a Physical Downlink Control Channel (PDCCH), a Physical Hybrid automatic repeat request Indicator Channel (PHICH), etc.
Definition of MIMO Technology
MIMO refers to a method capable of improving the efficiency of data transmission/reception using multiple transmission antennas and multiple reception antennas, instead of a conventional method employing one transmission antenna and one reception antenna. That is, MIMO is a technology utilizing multiple antennas in a transmitter or a receiver of a wireless communication system to increase capacity or improve performance. Here, MIMO is referred to as multiple antennas.
The MIMO technology is an application of techniques for restoring data by collecting pieces of data received through several antennas, without depending on a single antenna path, in order to receive a message. Since the MIMO technology can improve a data transmission rate in a specific range or increase a system range at a specific data transmission rate, it may be widely applied to mobile communication terminals, relays, etc. MIMO technology is drawing attention as a next-generation technology to overcome limitations in mobile communication transmission capacity, which is increasingly limited due to expansion of data communication.
FIG. 6 illustrates the configuration of a general MIMO communication system. As shown in FIG. 6, if the numbers of transmission and reception antennas are simultaneously increased to NT and NR, respectively, a theoretical channel transmission capacity is increased in proportion to the number of antennas, unlike the case where only either a transmitter or a receiver uses multiple antennas. Accordingly, it is possible to increase transmission rate and to remarkably improve frequency efficiency. Theoretically, a transmission rate according to an increase in channel transmission capacity can be increased by a value obtained by multiplying a rate of increase, Ri, indicated in the following Equation 1 by a maximum transmission rate Ro in case of using one antenna.Ri=min(NT,NR)  [Equation 1]
For example, in a MIMO communication system using four transmission antennas and four reception antennas, it is possible to theoretically obtain a transmission rate which is four times a transmission rate of a single antenna system. After an increase in the theoretical capacity of the MIMO system was first proved in the mid-1990s, various techniques for substantially improving data transmission rate have been actively developed. Several of these techniques have already been incorporated in a variety of wireless communication standards such as the 3rd generation mobile communication and the next-generation wireless local area network.
Active research up to now related to the MIMO technology has been focused upon a number of in different aspects, including research into information theory related to the computation of MIMO communication capacity in various channel environments and in multiple access environments, research into wireless channel measurement and model derivation of a MIMO system, and research into space-time signal processing technologies for improving transmission reliability and transmission rate.
Reference Signal Allocation Scheme in 3GPP LTE Downlink System
In the radio frame structure applicable to FDD out of the above-described radio frame structures supported by 3GPP LTE, one frame is transmitted during a 10 msec duration. One frame consists of 10 subframes each with a duration of 1 msec. One subframe consists of 14 or 12 OFDM symbols. The number of subcarriers selected in one OFDM symbol can be one of 128, 256, 512, 1024, 1536, and 2048.
FIG. 7 illustrates the structure of a UE-specific downlink reference signal in a subframe in which one Transmission Time Interval (TTI) uses a normal Cyclic Prefix (CP) having 14 OFDM symbols. In FIG. 7, ‘R5’ denotes a UE-specific reference signal and l denotes a position of an OFDM symbol on a subframe.
FIG. 8 illustrates the structure of a UE-specific downlink reference signal in a subframe in which one TTI uses an extended CP having 12 OFDM symbols.
FIGS. 9 to 11 illustrate the structures of UE-common downlink reference signals for systems having one, two, and four transmission antennas, respectively, when one TTI has 14 OFDM symbols. In FIGS. 9 to 11, R0, R1, R2, and R3 represent pilot symbols with respect to transmission antenna port 0, transmission antenna port 1, transmission antenna port 2, and transmission antenna port 3, respectively. No signals are transmitted in subcarriers where pilot symbols of the respective transmission antennas are used to eliminate interference with the other transmission antennas except for the transmission antennas transmitting the pilot symbols.
The downlink reference signals shown in FIGS. 7 and 8 may be simultaneously used together with the UE-common downlink reference signals shown in FIGS. 9 to 11. For example, in OFDM symbols 0, 1, and 2 of the first slot to which control information is transmitted, the UE-common downlink reference signals shown in FIGS. 9 to 11 may be used, and in the other OFDM symbols, UE-specific downlink reference signals may be used. If a predefined sequence (e.g. Pseudo-Random (PN) sequence, m-sequence, etc.) is multiplied by a downlink reference signal according to each cell before transmission, channel estimation performance in receiver can be improved by reducing interference of a signal of a pilot symbol received from a neighboring cell. The PN sequence is applied in units of OFDM symbols in one subframe. Different PN sequences may be applied according to a cell ID, a subframe number, an OFDM symbol position, and a UE ID.
As an example, it can be understood that, in the structure of a 1Tx pilot symbol shown in FIG. 9, the number of pilot symbols for one transmission antenna used in a specific OFDM symbol including a pilot symbol is two. The 3GPP LTE system includes a variety of bandwidths ranging from 60 RBs to 110 RBs. Accordingly, the number of pilot symbols for one transmission antenna in one OFDM symbol is 2×NRB and a sequence multiplied by the downlink reference signal according to each cell should have a length of 2×NRB. NRB denotes the number of RBs corresponding to a bandwidth and the sequence may use a binary sequence or a complex sequence. One example of the complex sequence is indicated as r(m) in the following Equation 2.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                                                                                                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                                                                                                                  ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ⁢                                  ⁢                  (                                                    where                ⁢                                                                  ⁢                m                            =              0                        ,            1            ,            …            ⁢                                                  ,                                          2                ⁢                                                                  ⁢                                  N                  RB                  max                                            -              1                                )                                    [                  Equation          ⁢                                          ⁢          2                ]            
In Equation 2, NRBmax represents the number of RBs corresponding to a maximum bandwidth and may be 110 according to the above description, and c represents a PN sequence and may be defined as a Gold sequence of length-31. In case of a UE-specific downlink reference signal, Equation 2 may be expressed by the following Equation 3.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                                                                                                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                                                                                                                  ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ⁢                                  ⁢                  (                                                    where                ⁢                                                                  ⁢                m                            =              0                        ,            1            ,            …            ⁢                                                  ,                                          2                ⁢                                                                  ⁢                                  N                  RB                  PDSCH                                            -              1                                )                                    [                  Equation          ⁢                                          ⁢          3                ]            
In Equation 3, NRBPDSCH represents the number of RBs corresponding to downlink data allocated to a specific UE. Therefore, according to the amount of downlink data allocated to a UE, the length of the sequence may vary.
Only one data stream can be transmitted through the above-described structure of the UE-specific downlink reference signal. Since the structure cannot be simply extended, it is impossible to transmit a plurality of data streams. Therefore, the structure of the UE-specific downlink reference signal needs to be extended to transmit a plurality of data streams.